This invention relates generally to methods for producing layers on substrates, and more particularly, to a method for continuously producing layers with intermetallic superconducting A15 phase niobium-germanium (Nb.sub.3 Ge) by the reduction of gaseous halogenides of niobium and germanium.
A method for producing layers of Nb.sub.3 Ge on a heated substrate by supplying a gas mixture containing hydrogen and the niobium and germanium halogenides to a reaction chamber through which an elongated substrate is pulled is described in "Journal of the Less-Common Metals", Vol. 59, 1978, pp. 35-41. A technique for preparing superconducting A15 phase niobium-germanium (Nb.sub.3 Ge) using chemical vapor deposition (CVD) technology is disclosed in "LEEE Transaction on Magnetics", Vol. MAG-14, No. 5, September 1978, pp. 608-610; and Vol. MAG-15, No. 1, January 1979, pp. 516-519. The technique described therein has been used for preparing the intermetallic superconducting compound niobium-tin (Nb.sub.3 Sn) as described in "RCA Review" Vol. XXV, September 1964, pp. 342-365. According to the techniques described in these references, intermetallic A15 phase Nb.sub.3 Ge can be precipitated by reduction of gaseous niobium and germanium halogenides, with hydrogen, on a heated substrate of illustratively copper or steel.
As noted in "Journal of the Less-Common Metals", the known production methods have not permitted the manufacture of superconductors which exceed approximately 20 meters in length. If such known methods are applied to the production of long conductor sections, illustratively, in excess of 100 meters, such long conductors will be characterized by a critical current density which drops steeply with increasing conductor lengths. Thus, although such conductors will have layer thicknesses and critical temperature characteristics which deviate over their length by less than 10%, the critical current density characteristic will decrease so as to be only 25% of the initial value at the end of a conductor ribbon which is 150 meters long. Such a decrease in the critical current density cannot be prevented even if the conductor is carefully prepared, and even if the germanium and niobium are deposited over deposition times of several hours with constant mass flow on a substrate which is transported with constant velocity through an atmosphere of a reaction gas mixture which is held at a constant temperature.
It is, therefore, an object of this invention to provide a method for precipitating superconducting A15 phase Nb.sub.3 Ge from the gaseous phase, onto conductors which exceed 100 meters in length, without incurring a steep drop in the critical current density characteristic as conductor length increases.